The first step in a number of microbiological analyses is the mechanical blending of the sample to be analyzed together with the mixing of the sample with a growth medium selected to be conductive to the rapid multiplication of the particular bacteria to be detected. For example, in the Salmonella detection procedure, sample amounts of from 25 to 375 grams commonly are blended mechanically with varying amounts of growth medium.
In practice, conventional food blenders comparable to those commonly sold for kitchen use are employed to accomplish the blending and mixing. The blending vessels used with these blenders for laboratory purposes are usually stainless stell or aluminum and are quite expensive. The blending vessels conventionally have a rotor assembly fastened in the bottom of the vessel which includes a vertical shaft extending through the bottom of the vessel. The shaft is held in a fluid-tight bearing or seal such that it may be easily rotated without leakage of the contents of the vessel. A cluster of blades or some other blending member is fastened to the inner end of the shaft and extends into the interior of the blending vessel. A coupling is usually located at the opposite end of the shaft, outside of the blending vessel, to allow the shaft to be removably engaged with the motor of the blender. As is well known in this art, the rotor that actuates the blending blades is to rotate at high speeds, such as 10,000 rpm, to produce the shearing action required to homogenize the solid materials that have been introduced into the vessel, with the liquid medium within the vessel.
Conventional blending vessels are substantially square in horizontal cross section. The square shape serves to break up the central vortex that tends to result from the rotation of the blades through the material contained in the blender. If the blending vessel were round in horizontal cross section, the contents would swirl about, forming a vortex driven by the blades. Such an arrangement would increase the chance for spillage if the blades were driven above a given speed and reduce the vertical migration of vessel contents so that materials in the upper part of the vessel might never come into contact with the whirling blades.
After being blended for a suitable length of time, the sample and the growth medium are reduced to a slurry. The slurry is then placed in a culture vessel that often contains an additional quantity of growth medium. When large amounts of sample are involved, more than one blending operation may be required owing to the limited capacity of the blending vessel. Conventional culture vessels are round in horizontal cross section in order to fit conveniently into common designs of centrifuges, bottle racks, and other lab equipment.
In order to avoid contamination, the entire blending and transfer process must be done under aseptic conditions. The blending vessel, culture vessel, and growth medium all must be sterilized before the procedure is begun. Almost invariably, blending and culture vessels are made of an autoclavable material and are sterilized by autoclaving. The growth medium may be autoclaved either separately, in a storage vessel, or in the blending or culture vessel in which it is to be used. The chance of undesirable microbiological contamination increases with every additional vessel involved in the total operation and with every transfer that must be made between vessels.
After blending and subsequent transfer to the culture vessel, a sample is incubated for a selected period of time whereafter subcultures are made and the detection procedure is continued. The growth medium and sample in the culture vessel may then be disposed of. Before disposal, it is necessary to sterilize the incubated growth medium because of the possible presence of dangerous microbiological species. Similarly, the blending vessel used at the beginning of the process also must be sterilized prior to washing to prevent any spread of the bacteria that may be present in the sample. While it is possible to use a bactericidal agent to sterilize the blending vessel, culture vessels and their contents are customarily autoclaved owing in part to the difficulty of reliably mixing a bactericidal agent with what is often a fairly large amount of growth media. Unfortunately, autoclaving is an inconvenient, time and energy consuming process, and is therefore expensive compared to the use of bactericides.